Many diseases, including cancers, arise from the abnormal over-expression or -activity of a particular gene, a group of genes, or a mutant form of protein. The therapeutic benefits of being able to selectively silence the expression of these genes is obvious.
A number of therapeutic agents designed to inhibit expression of a target gene have been developed, including antisense ribonucleic acid (RNA) (see, e.g., Skorski, T. et al., Proc. Natl. Acad. Sci. USA (1994) 91:4504-4508) and hammerhead-based ribozymes (see, e.g., James, H. A, and I. Gibson, Blood (1998) 91:371). However, both of these agents have inherent limitations. Antisense approaches, using either single-stranded RNA or DNA, act in a 1:1 stoichiometric relationship and thus have low efficacy (Skorski et al., supra). Hammerhead ribozymes, which because of their catalytic activity can degrade a higher number of target molecules, have been used to overcome the stoichiometry problem associated with antisense RNA. However, hammerhead ribozymes require specific nucleotide sequences in the target gene, which are not always present.
More recently, double-stranded RNA molecules (dsRNA) have been shown to block gene expression in a highly conserved regulatory mechanism known as RNA interference (RNAi). WO 99/32619 (Fire et al.) discloses the use of a dsRNA of at least 25 nucleotides in length to inhibit the expression of a target gene in C. elegans. dsRNA has also been shown to degrade target RNA in other organisms, including plants (see, e.g., WO 99/53050, Waterhouse et al.; and WO 99/61631, Heifetz et al.), Drosophila (see, e.g., Yang, D., et al., Curr. Biol. (2000) 10:1191-1200), and mammals (see WO 00/44895, Limmer; and DE 101 00 586.5, Kreutzer et al.).
Briefly, the RNA III Dicer enzyme processes dsRNA exceeding a certain length into small interfering RNA (siRNA) of approximately 22 nucleotides. One strand of the siRNA (the “guide strand”) then serves as a guide sequence to induce cleavage of messenger RNAs (mRNAs) comprising a nucleotide sequence which is at least partially complementary to the sequence of the guide strand by an RNA-induced silencing complex RISC (Hammond, S. M., et al., Nature (2000) 404:293-296). The guide strand is not cleaved or otherwise degraded in this process, and the RISC comprising the guide strand can subsequently effect the degradation of further mRNAs by sequence specific cleavage. In other words, RNAi involves a catalytic-type reaction whereby new siRNAs are generated through successive cleavage of long dsRNA. Thus, unlike antisense, RNAi degrades target RNA in a non-stoichiometric manner. When administered to a cell or organism, exogenous dsRNA has been shown to direct the sequence-specific degradation of endogenous messenger RNA (mRNA) through RNAi.
Gautschi et al. report that the expression levels of the anti-apoptotic proteins Bcl-1 and Bcl-xL are elevated during the development and progression of tumors (Gautschi, O., et al., J. Natl. Cancer Inst. (2001) 93:463-471). Tumor growth (but not size) was reduced by approximately 50-60% in nude mice treated with a combination of single-stranded antisense oligoribonucleotides targeted to Bcl-2 and Bcl-xL genes. However, because of the 1:1 stoichiometric relationship and thus low efficiency of antisense RNA, the anti-Bcl treatment required 20 milligrams of antisense RNA per kilogram body weight of recipient mouse per day. Producing therapeutically sufficient amounts of RNA is not only expensive, but single-stranded antisense RNA is highly susceptible to degradation by serum proteases, thus resulting in a short in vivo half-life.
Despite significant advances in the field, there remains a need for an agent that can selectively and efficiently silence a target gene using the cell's own RNAi machinery. More specifically, an agent that has both high biological activity and in vivo stability, and that can effectively inhibit expression of a target anti-apoptotic gene at a low dose, would be highly desirable. Compositions comprising such agents would be useful for treating diseases caused by the expression of these genes.